[libvirt] libvirt + xen 3.2.1 oddities

Daniel P. Berrange berrange at redhat.com
Wed Nov 26 20:56:28 UTC 2008

On Wed, Nov 26, 2008 at 11:14:51AM -0700, Jim Fehlig wrote:
> Daniel P. Berrange wrote:
> > On Fri, Nov 21, 2008 at 11:13:04PM +0100, Guido G?nther wrote:
> >   
> >> Hi,
> >> I just ran across these oddities when using a bit more libvirt+xen:
> >>
> >> 1.) virsh setmaxmem:
> >>
> >> On a running domain:
> >>         # virsh setmaxmem domain 256000
> >> completes but virsh dumpxml as well as the config.sxp still shows the
> >> old amount of memory. Looks as the set_maxmem hypercall simply gets
> >> ignored. xm mem-max works as expected. Smells like a bug in the ioctl?
> >>     
> >
> > The setmaxmem API is not performance critical, so it sounds like we
> > should first try setting it via XenD, and use Hypervisor as the
> > fallback instead.
> >   
> I have a patch for 0.4.6 in suse packages to do just this.  Using xend,
> you also get the value changed in dom config.

Do send the patch to the list & we'll see about applying it...

> >
> > This is basically a XenD bug. What's happening is that the domain
> > has been shutdown, and got most of the way through cleanup, as far
> > as the hypervisor is concerned. But something is still hanging around
> > keeping the domain from being completely terminated. In this case
> > XenD takes the dubious approach of just pretending the domain does
> > not exist. So libvirt sees it exists in the hypervisor, but when
> > asking XenD for more data, it gets that error. This really really
> > sucks.
> >   
> I spent some time looking into this bug as well.  I found that we ask HV
> for number of domains and get back more than actually exist.  We
> subsequently query xend about such domains and get the error message
> noted.  It turned out being a 'dead domain' memory leak in xen itself. 
> Jan Beulich plugged the hole and sent patch upstream but I can't seem to
> find the relevant c/s now :-(.  Anyhow, with Jan's fix I no longer see
> these error messages.

This seems to be quite a common problem for a number of users. Its good
that Xen has a fix now, but if switching to querying XenStore for domain
IDs make it work we should do that as a preventative measure in libvirt

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